Mercury button switch



May 1, 1962 l.. w. cooK 3,032,633

MERCURY BUTTON SWITCH Filed oct. 1, 1959 large head on the interior of the shell.

United States 3,032,633 Patented May 1, 1962 hcc 3,032,633 MERCURY BUTTN SWITCH Leonard W. Cook, Warwick, RJ., assignor to General Electric Company, a corporation of New York Filed Oct. 1, 1959, Ser. No. 843,863 3 Claims. (Cl. 20G-152) This invention relates to a mercury button switch; more particularly, the invention relates to a mercury button switch having internal structure to equalize pressures between two cavities and to reduce the maximum flow of current, thereby extending the service life of such switches.

In yCook and Passarelli Patent No. 2,916,589, which issued December 8, 1959, there was disclosed a mercury button switch in which an inner liner had a mercury reservoir arranged to impart increased kinetic energy to moving mercury upon rotation of the button and in which easily icnizable argon was added to the reducing hydrogen within the body of the button.

The present invention utilizes broadly the structure of the above-described Cook and Passarelli application but introduces certain refinements to some of the elements thereof. The Cook and Passarelli application describes a mercury button switch comprising a cylindrical metal shell having a closed terminal end and an open end upon which a cover plate is hermetically sealed. The cover plate includes a circular central section of insulating material axially pierced by a metallic terminal pin having a A liner in the interior has a central transverse partition which divides the interior of the shell into two cavities joined by a through-hole extending through the partition. One side of the partition is hollowed to form a reservoir and stiflicient mercury is placed inside the shell so that when the button is rotated to an On position pools of mercury in the two cavities will come into contact with each other in the through-hole thereby establishing an electrical circuit between the two terminals through the mercury. When the button is rotated to an Off position, the through-hole is raised above the surface levels of the separate pools of mercury and the reservoir is lowered beneath the surface of one of the pools of mercury. The cover plate has a rim of metal but a ring gasket prevents electrical contact between this rim and the mercury. The atmosphere surrounding the mercury is composed of hydrogen to which argon to promote ionization has been added.

Mercury is a relatively poor electrical conductor and it can establish a very rigorous environment when a circuit is completed through it in a mercury button switch. When two pools of mercury come together to close an electrical circuit or when the pools break to open a circuit, arcing conditions are established which vaporize a portion of the mercury explosively to create internal pressures estimated to be as high as 600 lbs. per square inch. This hot mercury vapor has been observed in a single cycle of operation to melt an .062 inch diameter hole in the steel shell of the button. Under such conditions, even ceramic insulating barriers are subject to deteriorative attack. Also, the preferred proportions of the two mercury pools are disrupted by this explosion.

As another example of conditions established on the interior of mercury button switches when circuits are completed therethrough, cut-away samples of the switch disclosed in the Cook and Passarelli application previously mentioned were subjected to intermittent cycles of 15 ampere tungsten lamp operation. When a tungsten lamp circuit is closed, there is an initial inrush of current of about ten times the steady state current, and, thus, a l5- ampere steady state lamp load would produce an initial inrush current of about amperes. When this occurred, the mercury pool of the cut-away sample buttons was observed to quiver and boil and frequently to are at the nail-head contact of the terminal extending through the cover plate. It was calculated that the peak current from the tungsten lamp circuit was 210 amperes (maximum alternating current) and since the contact area of the nail-head was 0.071 square inch, the maximum current density at the mercury-metal interface was calculated to be 8,400 amperes per square inch. While the test load was greater than that encountered, on the average, but within the electrical rating of the device, it is obvious from the above ligures that even a conventional residential lamp load can subject the terminal contact of the mercury button to boiling mercury temperatures, which may interfere with the dependable passage of current through the terminal.

The present invention has as `one of its objects to provide a mercury button switch in which internal stresses at the terminal connections to the mercury are reduced to a minimum.

Another object of the present invention is to provide a mercury button switch having a reliable operating life over a long period of time.

Another object of the present invention is to provide amercury button switch in which the explosive effect of arcing is minimized and internal pressures are equalized.

Briefly stated, in accordance with one of its embodiments, the present invention is directed to a mercury button switch comprising a cylindrical metal shell having a closed terminal end and an open end, a circular cover plate hermetically sealed over the open end of the shell, the cover plate having a metallic rim around an annular insulator, a terminal having an enlarged scored surface of large contact area on the interior of the shell axially piercing the annular insulator, an insulating liner in the shell, the liner including a central transverse partition dividing the shell into two cavities, the cavites `being joined by a through-hole through the partition vfor the passage of mercury and a vent hole through the partition for equalizing pressures in the cavities, one side of the partition being hollowed somewhat to provide a reservoir for mercury, a quantity of mercury in the cavities measured to provide two separate pools when the switch is in an Oli position and a single pool joined through the through-hole when the switch is rotated to an On position, the vent hole always being clear of mercur, and a sealing gasket between the periphery of the liner and the rim of the cover plate whereby an electrically conductive path is established between terminals through the terminal end of the cylinder, the mercury pool and' the enlarged scored surface of the cover plate terminal only when the switch is in an On position.

The invention will be better understood from the following descriptio-n considered in conjunction with the ac- FIG. 2 is a sectional view taken along the line 2 2 of FIG. 5;

FIG. 3 is an end view of the Vswitch from the left side as depicted in FIG. 2;

FIG. 4 is an exploded perspective view of the switch elements;

FIG. 5 is a cross-sectional View taken along the line 5 5 of FIG. 2 with the switch in On position; and

FIG. 6 is a sectional view similar to FIG. 5 except that the switch has been rotated to Oli position.

As best shown in FIG. 4, the principal shaped elements of the switch are a cylindrical metal shell 10, a liner 11 of insulating material, a gasket 12 of high-temperaturestable insulating, ma-terial, and a cover plate 13 which includes an annular insulator 14 pierced by a terminal 1S having an enlarged head with irregular surface 16.

The cylindrical shell has an axially extending ernbossment 17 which serves as a terminal and bearing surface for rotation of the switch. As shown in FIG. 3, next to the terminal embossment 17 is another embossment 18 in alignment with a through-hole or passageway in the liner 11, the embossment 1S providing an inclined surface to redirect an arc formed during operation of the switch through an extended path of cooling gas atmosphere. The cylindrical shell 10 also has an indentation 19 which mates with a depression on the liner 11 thereby helping to position the liner 11 within the cylindrical shell 10. Indentation 19 mates with a corresponding indentation 32 in the liner 11. A portion of the periphery of the cylindrical shell 1t) has an indexing slot 21 which mates with a corresponding slot 22 on the liner 11 to serve the dual function of positioning the liner 11 with respect to the cylindrical shell 10 and the cylindrical shell 10 with respect to an external rotating mechanism (not shown). The open end ot' the cylindrical shell 10 has a tlange 23 to provide a base for hermetically sealing the cover plate 13 to the cylindrical shell 1t) as by welding the rim of the cover plate 13 to the flange 23.

The liner 11 is composed of a high-temperature-stable insulating material and in the present invention diiers considerably from the liner described in the Cook and Passarelli patent previously mentioned. It is preferred that the liner 11 be composed of a porous or vitreous ceramic such as porcelain, steatite, or aluminum oxide ceramics. The liner 11 has a central transverse partition 24 pierced by a large through-hole or passageway 25 into which two pools of mercury i'low to make contact when the switch is closed and a smaller Ventilating passageway 26 which is above the mercury pools both in On and Oft positions of the switch and which serves to equalize pressures in a pair of cavities 27 and 28 inside the cylindrical shell 10 which are divided by the partition 24.

The partition 24 has been hollowed out ysomewhat on the cavity 27 side to provide a reservoir 31 capable of holding a relatively large quantity of mercury when the button is rotated to Oli position. In order to increase this capacity, the periphery of the liner 11 outside of the reservoir 31 is open to the cylindrical shell 10. As shown in FIGS. 5 and 6, the liner 11 has an indentation 32 which mates with the indentation 19 of the cylindrical shell 10 in order to help maintain the orientation of the cylindrical shell 10 and liner 11. It will be noted that the indentation 32 is directly opposite the slot 22 in order to provide the most etiicient positional support for the elements positioned.

ri'he gasket 12 forms an insulating seal between the liner 11 and metal rim of the cover plate 13, as best shown in FIG. 2. A preferred material for the gasket 12 is a molded silicone polymer but other high-.temperature-stable materials such as polytetrafluoroethylene can be used. It is desirable that residual organic materials be removed from the gasket 12 by prolonged heating at a temperature just below that at which damage would occur to the gasket. In the case of commercially available silicone rubbers, treatment for 24 hours at a ternperature of about 480 F. is sufficient to remove con- All taminating volatile constituents which might otherwise be gradually released during operation of the switch to contaminate the pure, active pools of mercury.

The metal rim, annular insulator 14, and terminal 15 of the cover plate 13 rnust be sealed together in gastight relationship. It is preferred that vitreous ceramics be used for this purpose. In order to increase the contact area between the mercury and the enlarged head 16 of the terminal 15--thereby reducing current density at this interfacethe head 16 is scored in order to provide additional contact area. With four parallel grooves or indentations 33 (FIG. 4) in the enlarged head 16, the maximum current ow through the interface when subjected to the test previously mentioned was reduced from 8,400 amperes per square inch to 5,000 amperes per square inch current density, a 40% reduction.

As shown in FIGS. 2, 5, and 6, a measured quantity of mercury 3d is added to provide a mercury level such that the through-hole 25 is free of mercury with the switch in an Oi position, as shown in FIG. 6. In the posi-tion of FIG. 6, the mercury is divided into two separate pools (FIG. 2) in the cavities 27 and 28. The reservoir 31 stores the major portion of the mercury in the cavity 27 and when the switch is now rotated to the On position of FIG. 5, the repositioning of the reservoir 31 imparts considerable kinetic energy to the mercury 34 previously in the reservoir 31 and this mercury rushes into the through-hole Z5 to make contact therein with the pool of mercury from the cavity 28. At higher current densities, this contact produces quite `an arc which vaporizes a portion of the mercury and creates unequal pressure conditions within the cavities 27 and 28. Pressure equalization cannot be provided within the throughhole 25 since the mercury and vapor therein are in a highly disturbed state. This equalization can take place through the vent hole 26 as this vent hole is always positioned above the level of the mercury pools.

As in the case of the Cook and Passarelli patent previously mentioned, the space within the cavities 27 and 28 not occupied by mercury is occupied by a mixture of hydrogen and argon in order to provide a reducing and ionizing atmosphere.

Mercury button switches constructed in accordance with this invention have been very successful in passing severe tests. Tests so severe that about half of the mercury switches of the prior art yfailed have produced failure in no more than 5% of the switches constructed in accordance with the present invention. While the invention has been described with reference to a particular embodiment thereof, it is obvious that there may be modiiications which will still fall within the true spiritv of the invention. Therefore, the invention should be limited in scope only as may be necessitated by the scope of the appended claims.

What I claim as new and desire -to secure by Letters Patent of the United States is:

l. In a mercury button switch, a cylindrical metal shell having a closed terminal end and an open end, a circular cover plate hermetically sealed over the open end of said shell, said cover plate having a metallic rim around an annular insulator, a terminal having an enlarged surface on the interior of said shell axially piercing said annular insulator, an insulating liner in said shell sealed at one end to said annular insulator, said liner including a central transverse partition divinding said shell into two cavities, said cavities being joined by `a through-hole through said partition for the passage of mercury, the side of said partition opposite said annular insulator defining a reservoir for mercury, said liner having its peripheral surface open to said shell at said reservoir, and a quantity of mercury in said cavities measured to provide two separate pools when said switch is in an Off position and a single pool joined through said through-hole when said switch is rotated to an On position whereby an electrically conductive path is established between terminals through 5 the terminal end of said cylinder, the mercury pool and the enlarged surface of the cover plate terminal only when the switch is in an On position.

2. A mercury button switch as claimed in claim 1 wherein the enlarged surface of the terminal consists of a plurality of parallel indentations.

3. A mercury button switch as claimed in claim 1 wherein the liner partition has a vent hole positioned to be always above the mercury.

References Cited in the iile of this patent UNITED STATES PATENTS Phelan Aug. 26, 1930 Spencer July 2, 1940 Satern Jan. 28, 1941 Bear Aug. 5, 1941 Mattern Feb. 22, 1944 Cook et a1.. Dec. 8, 1959 

